CS 105: TOPIC 12 DICTIONARIES Max Fowler (Computer Science) - PowerPoint PPT Presentation

CS 105: TOPIC 12 – DICTIONARIES Max Fowler (Computer Science) https://pages.github-dev.cs.illinois.edu/cs-105/web/ JULY 26, 2020

Week 7 Video Series Topics  Dictionaries  A reminder on Key vs Value  Advanced dictionary use – histograms and get  Advanced dictionary use – update

Dictionary – Key vs Value Reminder

A reminder of dictionary structure  You’ve been using dictionaries for a while now.  animals_on_island = {“puffin”:20, “alpaca”:25, “cats”:10}  Key – animals  Value – The population of that animal

Print the total number of animals on the island  Effectively, sum the values in the dictionary.  Can we just do….: sv = 0 for val in animals_on_island: sv+= val

Variable names aren’t magic  ‘ val ’ has no meaning to Python – it is just an identifier for a variable  To get access to the values, we need to do one of three things:  1) Use the keys  2) Use .values()  3) Use .items()

Approach one – using keys to access values def animal_pop(animals_on_island): total_pop = 0 for animal in animals_on_island: total_pop += animals_on_island[animal] return total_pop

Approach two – using .values()  .values() is a function that lets us get the values in a sequence def animal_pop2(animals_on_island): total_pop = 0 for pop in animals_on_island.values(): total_pop += pop return total_pop

Approach three – using .items()  .items() gives us both the key AND the value for each pair – so we need two variables def animal_pop3(animals_on_island): total_pop = 0 for animal, pop in animals_on_island.items(): total_pop += pop return total_pop

Video question  Assuming my_list is a list of strings and my_dict is a dictionary with int keys and string values, what does this code do? s = 0 for key, val in my_dict.items(): if val in my_list: s += key print(s)

Advanced Dictionary – get

Dictionaries – common paradigm  Given a list/file of something, make a histogram of the items in the list  What’s a histogram? A distribution  A dictionary of values and their counts

Problem – updating counts.  Given histogram = {“apple”:5, “pineapple”:2”}  histogram[“apple”] += 1 #Updates to six  histogram[“orange”] +=1  #A key error – orange doesn’t exist yet!!!

Getting around the key issue – long way histogram = {} for item in a_list: if item in histogram: histogram[item] += 1 else: histogram[item] = 1

Dictionaries - get  get is a method of dictionaries that simplifies this process  How does get work?  a_dictionary.get(key, default)  If key exists, get the value  If the key is not in the dictionary, use the default value provided

Simplified down using get histogram = {} for item in a_list: histogram[item] = histogram.get(item, 0) + 1  Notice…  We use 0 as the default, so we can always “+1”  No more if statement!!  No += -> have to use regular assignment

Video question – what does this code do? def fn(other_dict, data_dict): for key in data_dict: other_dict[key] = other_dict.get(key,0) + data_dict[key]

Advanced Dictionary – update

Dictionary updates  In the last video question, we updated a dictionary using the key-value pairs from another  Dictionaries have a method for that, but there is a catch… update()

update  a_dict.update(another_dict)  For each key in another_dict …  If the key does not exist in a_dict, add it  If the key DOES exist in a_dict, CHANGE THE VALUE

Use cases for updating dictionaries  Use a loop and get IF  You want to add/subtract/somehow combine values for existing keys  Use update if  You want to update the dictionary, erasing old data, OR the two dictionaries do not share any keys

Video question  Is this best solved with the use of .update() or with a loop and .get()?  “Write a function which takes two dictionaries – old and new. It should use the new dictionary to update the keys in old, replacing any existing values and adding any new keys”